Abstract

We study the physical conditions in damped Lyman-? systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H 2 by Ledoux et al. (2003). H 2 is detected in 13-20% of the DLAs in our sample. Using the rotation level populations of H 2 and fine-structure excitations of C I we show the mean kinetic temperature of H 2 components is 153±78 K, n H = 10-250 cm -3, and the ambient radiation field is similar to or slightly higher than the mean diffuse UV field of the Galaxy. Combining this with the success rate of detecting H 2 in DLAs we conclude that at least 13-20% of DLAs at z abs > 1.9 show the presence of CNM and substantial star-formation activity. C II* absorption is detected in all the components where H 2 absorption is seen. The level populations of C II in these systems is consistent with the physical parameters derived from the excitation of H 2 and C I. We detect C II* in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H 2. The absence of C I absorption, the measured N(C II*)/N(C II) and N(Al III)/N(Al II) ratios in these systems are consistent with the gas having lower density (n H= 0.3-6 cm -3) than that seen in the H 2 components. 50% of the DLAs that do not show C II* are consistent with them originating from a low density warm neutral medium. © 2005 International Astronomical Union.